The present invention relates to a sheet metal fin for use in a heat exchanger, and also relates to a heat exchanging system having such sheet metal fins.
Fins in the form of metal sheets provided with oval passages for receiving oval heat exchanging pipes are generally known in heat exchangers, particularly in radiators for motor vehicles (EP 0176 729). Each of the passages in the fin includes a drawn through collar whose height is constant over the entire periphery of the passage or at most fluctuates within standard tolerance range. An optimum height of the collars is usually determined experimentally because by increasing the height of the collars above a certain level no substantial increase in efficiency of the heat exchanger is achieved whereas collar heights below the optimum height leads to a distinct lowering of efficiency. The drawn down collars are manufactured in such a way that in a first punching step a plurality of openings is punched out in the sheet metal fin by means of draw punch and a die having cutting edges; subsequently in a second deep drawing step the oval collar is shaped by means of a drawing punch and a further drawing die.
Heat exchanger provided with fins of the above described kind represent a special type of pipe radiators. They distinguish from conventional pipe radiators primarily due to the fact that the heat exchanging pipes are connected to the sheet metal fins solely by expansion of their cross-section without any additional soldering, welding or glueing of the pipes to the edges of the corresponding passages. In order to achieve a good thermal efficiency it is necessary that the walls of the pipes always fully contact the inner wall of the collars.
When using pipes of oval cross-section whose ratio of the maximum diameter to the minimum diameter is relatively small, for example 2:1, there are no problems in accomplishing a perfect connection. However in the case of extremely flat oval cross-sections of the pipes wherein the ratio of the two diameters is larger for example 3:1 through 8:1, it is necessary to provide increased heights of the collars. Due to the dimensions of the collars, the increased height during the deep drawing step leads to an expansion of the sheet metal material by 200% and more which reach and frequently exceed the tensile strength of the collars. As a consequence, in order to reliably prevent the crack formation during the drawing of the collars, the height of the collars is less than the optimum value. This in turn causes an undesirable reduction of efficiency. Alternatively, attempts must be made to prevent the formation of cracks in the processed collars by using a special quality of the sheet metal material. This possibility, however, would increase material costs on the one hand, and would not insure with certainty that during subsequent expansion of the pipes or even during the following use of the heat exchanger, the collars would not crack due to mechanical vibrations, hydraulic pressure of the cooling fluid, thermal expansions, coupling contraction and the like, on the other hand. Since cracks in the collars diminish not only the efficiency of the heat exchanger but also substantially reduce the circumferential tension in the collar necessary for establishing a proper contact with the heat exchanging pipe, the mass production of sheet metal fins for extremely flattened oval heat exchanging pipes and thus of the final heat exchangers still represents an excessive safety risk as long as, for achieving a high efficiency of the heat exchanger, the optimum height of the collar is to be achieved.